Whereas, in a conventional inkjet printer, the print heads which are mounted on a carriage spray ink droplets line by line in the transverse direction onto the medium which is transported discontinuously in the running direction, in a single-pass inkjet printer the print heads are mounted in print head modules of the type mentioned in the introduction over the entire width of the medium in the transverse direction. The printing medium can be moved continuously in the running direction. Whereas printing speeds of up to 2 m per minute are reached in a conventional inkjet printer, printing speeds of up to over 50 m per minute are achieved by way of a single-pass inkjet printer. For color printing, a plurality of print-head modules are mounted behind one another in the running direction in a single-pass inkjet printer. Here, the print-head modules are assigned in each case one primary color, in particular cyan, magenta and yellow and possibly black. For special printing uses, print-head modules having a special color can be added.
A single-pass inkjet printer is suitable, in particular, for industrial use, in which high-quantity goods have to be printed and a high throughput is therefore important. On account of the high printing speeds, a single-pass inkjet printer is likewise suitable for printing objects with a large surface area. A single-pass inkjet printer is therefore suitable, in particular, for industrial applications of the furniture or ceramic industry, where floor coverings, such as laminates or ceramic tiles, counter tops, profiled strips or the like are to be provided with a decorative pattern. Here, a very wide variety of inks are used which are, for example, resistant with respect to a subsequent protective coating, etc.
In comparison with conventional printing processes, such as gravure printing or the like, the single-pass inkjet printer is also used precisely in the case of small production batches, where the production of an impression roll is not worthwhile. In contrast, a single-pass inkjet printer also makes individualization of the decorative patterns possible, and also impossible decorative patterns which cannot be achieved by way of rolls. The single-pass inkjet printer is not restricted to a constant repetition of a printing pattern or repeating pattern, as is the case in rotary printing.
A print-head module for a single-pass inkjet printer can certainly reach dimensions of more than half a meter up to over a meter in the transverse direction and vertically. The individual print heads which are combined in the print bar of a print-head module can have widths of up to several tens of cm. Here, resolutions of up to 600×600 dpi (dots per inch) are possible. Here, several thousand nozzles are contained per print head. Printing widths of up to several meters can be achieved by way of large print-head modules or by way of the arrangement of a plurality of print-head modules next to one another.
Positional deviations of a few micrometers are visible to the human eye in a printed image. In the case of the abovementioned resolutions, the individual nozzles of a print head lie only several tens of μm apart from one another. The size of an image point is itself in the region of 10 μm. In a single-pass inkjet printer having a plurality of print-head modules which are arranged behind one another in the running direction, it can be seen that an adjustment of the print heads in the micrometer range becomes necessary, in order to produce a high-quality printed image. The adjustment of a print-head module in a single-pass inkjet printer is therefore complicated. The position of the print heads has to be detected, for example, by optical microscope and has to be set manually in a complicated way. The setting up of a single-pass inkjet printer is thus comparatively protracted. An adjustment also has to be carried out for each replacement of a print-head module. This leads to an unnecessary extension of the down times.